Particulate methane monooxygenase, pMMO, catalyses the oxidation of methane to methanol using dioxygen. In this study, M. albus BG8 membrane fractions are treated with ferricyanide. One advantage of M. albus BG8 is that only pMMO is present and no soluble sMMO with the oxo-bridged dinuclear iron center is present. Difference spectra from pMMO membrane fractions treated with and without ferricyanide give two signals. One of the signals, referred to as signal Z, is a broad, more isotropic signal centered at g=2.13. The other signal, referred to as signal A, has g-parallel=2.24 and g-perpendicular=2.13 and is the signal observed by others. If samples with signal Z are treated with higher concentrations of ferricyanide or for prolonged periods of time with ferricyanide, more signal A and less signal Z are found in the EPR spectra. Similarities and differences between signal A and the mixed valence binuclear copper [HIS-N-Cu(1.5+)-(CYS-S-)2-Cu(1.5+)-N-HIS] center in nitrous oxide reductase and cytochrome c oxidase and between signal Z and the Z-center in nitrous oxide reductase are discussed.